This project has as its objectives to learn about: (1) The mechanism by which the thyroid gland maintains a concentration of iodide elevated above that of the blood, and (2) The mechanism by which the thyroid gland forms and accumulates protein-bound iodine. The work is largely based upon in vivo studies and makes use of autoradiography, cytochemistry at ultrasructural level, kinetic studies, and inhibitors of steps in the synthesis of thyroid hormones.